After Effects :: RAM Preview Not Playing Back Realtime FPS In CC

I'm having trouble with AE CC playing a RAM preview at a realtime framerate - it's lagging behind at random speeds. This is a short image sequence (of many more to come) My iMac just seems to struggle with this, I can't figure out why. Is this a Graphics card issue?? We have an older mac here which has an older card with more ram (1024 MB)  on the  card, which takes longer to render the preview (is that due to less system RAM?) But, plays back with a solid 30fps framerate no worries..Is this a problem with the particular Graphics Card on this iMac? There seems to be little info about this card that I can find with After Effects.
 
Project task:
 
- I am  only previewing a short (15 sec) 1920 X 1080 image sequence (either TIFF or EXR) at 30 FPS. Ram preview going at around 20-28 fps: 'fps: 20.271 / 30.00 NOT realtime' 
- RAM Preview set to Full Res, 30 fps Frame Rate, Composition settings 30fps, footage 30fps
- tried lower quality settings, tried cropping video - same jerky framerate
- Even when video is rendered to a .Mov Lossless file the video is still juddery and appears to be dropping frames when played in Quick Time - VLC is better slightly.
 
System / Memory Preferences:
 
Ram shared by other apps: 4GB Ram reserved for AE: 28GB..After Effects Multiprocessing: ON Installed CPUs: 4 CPUs reserved for other apps: 2 RAM allocated per bg CPU: 6GB actual CPUs used: 2
 
Extra notes:
 
- Have tried with using less memory on the system, no diffeence - not a problem with the RAM in the iMac (which is above the recommended standard, I know, but works fine with everything)
- Running After Effects CC - no other programs running
- Works fine with other Mac we have - which is OSX Lion & After Effects 5.0 with ATI Radeon 5750 HD 1024MB Graphics Card.
- Had the same issues with After Effects 5.5
- Tried with After Effects 5.5 with Apple RAM 12GB no difference on this machine
- Monitored System CPU & GPU - no problem running at  CPU 25% & GPU 40-50%